1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer and the like, and more particularly to an improvement in an image forming apparatus in which image forming on a recording material having a long width is possible.
2. Description of the Prior Art
Explaining this type of the conventional image forming apparatus by taking, for example, an electrophotographic system as an example, there has been the one provided with a latent image writing device for writing an electrostatic latent image to a photoreceptor drum as an image-bearing member, a developing device for making this electrostatic latent image visible and a transferring device for transferring this developed image onto a sheet as a recording material.
In this type of image forming apparatus, an adopted latent image writing device is provided with a charger for charging the photoreceptor drum and an exposure device for writing the electrostatic latent image onto the charged photoreceptor drum.
This type of the image forming apparatus is in many cases provided with plural accommodating trays in which various types of sheet are accommodated for enabling the image formation to a sheet of various size or to sheets of large capacity. In this case, non-uniformness exists in each accommodating tray due to a mechanical tolerance, so that the position for accommodating the sheet varies for every tray in the widthwise direction (corresponding to the direction perpendicular to the advancing direction of the sheet).
If the latent image writing position by the exposure device is adjusted in accordance with the position of the sheet accommodated in every accommodating tray, the positional displacement of the sheet involved with the mechanical tolerance of the accommodating tray can be absorbed, thereby enabling to set such that the central position of the image matches to the central position of the sheet supplied from each accommodating tray.
The adjusting technique of the image writing position described above has been widely known including a technique for adjusting a position of a recording head described in, for example, Japanese Published Unexamined Patent Application No. Hei 5-281814.
In this type of the image forming apparatus, the adjusted range of the latent image writing position by the exposure device requires to be capable of absorbing the mechanical tolerance of the accommodating tray. In this case, it is inevitable that the effective developing width is determined to also consider the adjusted range of the latent image writing position in addition to the range where the image formation is possible.
On the other hand, in the latest image forming apparatus, it has already been proposed that an image of A3-size and a register mark around the image for the sheet cutting position can be written to a sheet having a long width of, for example, 12.6 inches (320 mm).
In order to enable the image formation to this type of long-sized sheet, it is necessary to secure an image area having a width of 305 mm as a whole, since the register mark should be given in addition to the image of A3-size on the sheet.
In order to correct the latent image writing position such that the central position of the image matches to the central position of the sheet, the adjusted range of the latent image writing position should be set for absorbing the mechanical tolerance of the accommodating tray. Assuming that the adjusted range of the latent image writing position is set to ±2 mm in this case, the required effective developing width of the developing device is a minimum of 309 mm. Therefore, the width of the developing device should be more enlarged than necessary, thereby entailing a technical problem of not meeting the demand for miniaturization of the developing device.
The present inventors have given attention to the image formation to the sheet having a long width, and has found that the mark such as the register mark given to the vicinity of the peripheral edge of the sheet is used only for the mark for later cutting the sheet, not remaining semipermanently in most cases. Considering this factor, we have found that there is less need to position the entire image area including even the mark such as the register mark with the central position of the sheet defined as a reference in the sheet having the long width.